Practice app/feature/get enrolled lessons endpoint 275

[egemenatikk]

This PR includes the following actions:

1. Check whether the incoming data is valid or not
2. Check whether a user with the given user_id exists or not.
3. Return enrolled lessons array of the user with the given user_id
4. Return an error message if an error occurs in any of the previous steps
5. Test the endpoint with Postman
6. Save requests as a collection in Postman
7. Document the endpoint

Now, the get enrolled lessons endpoint is ready for requests.

This PR closes #275, other details can also be seen from that issue.

[surmelienes1]

It seems very good and complete, thank you @egemenatikk for your valuable contributions.

I will now merge these changes with the master branch and automatically close the related issue #275 since I don't have anything to further add, if you have any questions, suggestions, or ideas, you can reopen the issue #275 and add your thoughts as a comment.